The scientific and technological issues of nanostructured particles and materials are currently attracting considerable attention. The small size of nanoparticles (generally used to indicate particles less than 100 nm), which can be responsible for the different properties (electronic, optical, electrical, magnetic, chemical, and mechanical) of nanoparticles and nanostructured materials with respect to the bulk material, makes them suitable for new applications. Nanosized powders have been synthesized by a number of processes including colloidal precipitation, mechanical grinding and gas phase nucleation and growth. Most synthesis methods of nanoparticles in the gas phase are based on homogeneous nucleation in the gas phase and subsequent condensation and coagulation. The gas phase synthesis route (aerosol route) makes it possible to generate new nanoparticles and nanostructured new materials from, in principle, a nearly unlimited variety of starting materials. However, two challenges need to be addressed for nanoparticles produced by aerosol process in order to be suitable for various applications, namely, (1) controlled size distribution of the primary particles, and (2) degree of aggregation, which has a direct effect on dispersibility. For most of the applications, it is very difficult to obtain the desired properties when the nanoparticles employed are either widely distributed in primary particle size or highly aggregated or both. Therefore, it is important to control the process parameters such as pressure, temperature, and concentration that aid in the determination of the properties of the resulting particles.
Jet expansion is a convenient fluid mechanical configuration for the controlled generation of ultrafine particles by gas-to-particle conversion. Condensable vapor can be introduced into the jet by evaporation from a solid or liquid into the gas upstream from the jet, or by chemical reaction in the jet. The jet configuration permits particle production with high throughputs under controlled conditions of temperature and dilution. U.S. Pat. Nos. 5,935,293 and 5,749,937 to Detering et al., teach a fast quenching reactor and method for thermal conversion of reactants to desired end products such as solid particles. The rapid quenching was achieved by adiabatic and isentropic expansion of gases in a converging-diverging nozzle. By converging-diverging is meant a nozzle whose area changes in the axial direction, first reducing (“converging”) to a minimum (“throat”), then increasing (“diverging”). Under sufficiently high pressure gradients, the flow velocity will increase with axial location, reaching a Mach Number of 1 at the throat and increasing to greater than 1 in the diverging section. The expansion taught can result in cooling rate exceeding 1010° C./s, thus preserving reaction products that are in equilibrium only at high temperatures. U.S. Pat. Nos. 5,788,738 and 5,851,507 to Pirzada et al., teaches similar approaches to the production of nanoscale powders by ultra-rapid thermal quench processing of high-temperature vapors through a boundary-layer converging-diverging nozzle, which is an adiabatic expansion process. The vapor stream is rapidly quenched at rates of at least 1,000° C./s, preferably greater than 106° C./s, to inhibit the continued growth of the nucleated particles and produce nanosize powder of narrow size distribution. One common feature of Detering et al., and Pirzada et al.'s work is that the sole purpose of employing a nozzle that is of a converging-diverging shape is to achieve rapid quench that is at least greater than 1,000° C./s, preferably greater than 106° C./s by hypersonic nozzle expansion.
U.S. Pat. Nos. 5,874,134 to Rao et al., teaches a method of producing nanostructured material by hypersonically expanding a particle-gas mixture through a convergent nozzle and directing the resulting jet against an impaction substrate. Similar work has been described where nanosize particles with a narrow size distribution were generated by subsonically expanding thermal plasma carrying vapor-phase precursors through a convergent nozzle of a similar shape.
A serious difficulty with the jet expansion as taught in the prior art is that these techniques require large pressure gradients to accelerate the flows; necessitating large and expensive pumps. All the aforementioned nozzles are operated at downstream pressure (gas pressure exiting the nozzle) lower than 760 torr, often considerably lower.
In addition, the discharging of hot gas into an open domain, such as a plasma gas entering a reaction chamber, results in a jet that will entrain local fluid, causing a recirculation region. Any reacting gas or particles so entrained in the recirculation zone will be exposed, possibly on multiple occasions, to the high temperature gas. This may greatly accelerate aggregation, sintering and coalescence of the particles, all of which are generally undesirable. Although not all of the reactant gas and particles may be entrained in the recirculation region induced by the hot gas discharge, the agglomerates formed during recirculation will enhance agglomerate formation downstream of the recirculation region through Brownian and turbulent collisions. The surprising advantages achieved in separating the location of the reactant inlets upstream of the point where hot carrier gas and the reactants gas come in contact with one another results in this point of contact being downstream of the recirculation in the region. The prior patent literature in this area has failed to teach the novel process results that can be achieved through the simple nozzle design of the present invention.
Presently, the teachings in the patent literature consider nozzle flow only in the thermodynamic sense; i.e., that the accelerating flow in the nozzle, when there is a sufficient pressure drop through the nozzle (which is controlled by the exit pressure), leads to lower dynamic temperature and pressure, hence leading to lower collision and coalescence rates. The issue here is not the nozzle per se, but the large pressure drop required for such flow, which can be expensive to maintain and difficult to scale. For example, accelerating the flow to a Mach number of 1, which requires a pressure drop of nearly a factor of 2, reduces the gas temperature to 75% of the stagnation temperature. This is generally the case for γ˜5/3, where γ is defined as the ratio of the specific heat at constant pressure to the specific heat at constant volume. The ratio of specific heat at constant pressure to the specific heat at constant volume, γ, is usually between 7/5 for diatomic gases and 5/3 for monatomic gases. Further cooling requires supersonic flow with substantially greater pressure drops. Since the temperature drop comes from isentropic adiabatic cooling, special precautions must be taken during the quench step to avoid recovering the temperature when slowing down the particles to subsonic velocities. The present invention demonstrates that nozzle-type flow can be used to produce nanosized particles without the need for thermodynamic cooling; the nozzle is operating under nearly isobaric conditions, which can be defined thermodynamically as the pressure ratio between the exit and inlet of the nozzle being less than 0.85, leading to Mach numbers of under 0.40.
Therefore, the present invention satisfies a need of developing a cost-effective high temperature aerosol process that is capable of making various types of nanopowders of narrow size distribution. The inventors have accomplished their desired result to invent a cost-efficient reactor and process that produces nanoparticles of the above described narrow size distribution for a variety of materials by controlling the fundamental fluid dynamics in the reactor, especially in the high temperature region, taking into consideration the recirculating flow and turbulent diffusion that may occur in the region between the hot gas inlets) and the reactants inlet(s). Thermodynamic cooling as described in the patent literature can be used in conjunction with this invention to further improve the particle size distribution.
The main objective of this invention relates to a high temperature apparatus (aerosol reactor) useful for producing nanoparticles that are easily dispersed (with a small degree of aggregation, less than 50 primary particles in an aggregate after the dispersion step, with primary particles that are narrowly distributed in size of about 10 nm and 100 nm, preferably between 10 nm and 50 nm and a BET surface area equal to or greater than about 10 m2/g). Nanoparticles are formed by injecting the reactants into a high temperature reaction chamber, followed by vapor phase reaction, gas-phase nucleation and subsequent particle growth by condensation and coagulation. The reaction zone contains a unique reaction chamber that is precisely designed to reduce gas and particle entrainment in the reactant inlets region and to promote efficient mixing in the region downstream of the reactant inlet(s). These features are the key to produce less aggregated nanoparticles with narrow size distribution.
This apparatus can be used for producing novel nanoparticles and nanophase materials by a high temperature aerosol process either with or without a chemical reaction using any type of energy source.